Driven by climate change, global mean sea level rose 11–16 cm in the twentieth century1,2. Even with sharp, immediate cuts to carbon emissions, it could rise another 0.5 m this century3,4,5,6,7,8,9,10,11,12. Under higher emissions scenarios, twenty-first-century rise may approach or in the extremes exceed 2 m in the case of early-onset Antarctic ice sheet instability4,8. Translating sea-level projections into potential exposure of the population is critical for coastal planning and for assessing the benefits of climate mitigation, as well as the costs of failing to act.
Land topography and elevation, as represented by DEMs, lie at the foundation of such translation. High-accuracy DEMs derived from airborne lidar are freely available for the coastal United States, much of coastal Australia, and parts of Europe, but are lacking or unavailable in most of the rest of the world. By contrast, SRTM is a near-global satellite-based DEM covering latitudes from 56 south to 60 north and thereby land home to 99.7% of world population (based on 2010 Landscan data13). It is the standard choice for extreme coastal water level (ECWL) exposure analysis covering areas where high-quality elevation data are unavailable or prohibitively expensive14,15,16,17,18,19,20,21.
SRTM models the elevation of upper surfaces and not bare earth terrain. It thus suffers from large error with a positive bias when used to represent terrain elevations. This is especially true in densely vegetated and in densely populated areas22,23,24,25. Mean error in SRTM’s 1–20 m elevation band is 3.7 m in the US and 2.5 m in Australia when using DEMs from airborne lidar as ground truth26. Spaceborne lidar from NASA’s ICESat satellite27, a sparser, noisier and less reliable source of ground truth than airborne lidar, indicates SRTM has a global mean bias of 1.9 m in the same band26. This degree of error leads to large underestimates of ECWL exposure28, and exceeds projected sea-level rise this century under almost any scenario3,4,5,6,7,8,9,10,11,12.